Hudini,
How does a Hainan pilot fare with regards to his CAAC license when he leaves the company?
It appears that expat pilots elsewhere don't get to keep their certificates, and are required to surrender them for cancellation before leaving China. Going to another Chinese airline requires a letter of release from the first employer and a re-issuance of the pilot certificate.
Re-issuance may mean re-taking the written and checkride.